In the article from yesterday we talked about PostgreSQL HyperLogLog with some details. The real magic of that extension has been skimmed over though, and needs another very small article all by itself, in case you missed it.
*Which Set Operation do you want for counting unique values?* The first query here has the default level of magic in it, really. What happens is that each time we do an update of the HyperLogLog hash value, we update some data which are allowing us to compute its cardinality.
